Hi Cigar Havana,
a quote from my manual ('87 420), page 62:
The oil pressure at idle speed may drop to .3 bar (4.4psi) if the engine is at operating temp. This will not jeopardize its operational reliability. Pressure must, however, rise immediately upon acceleration.
Based on that, you are still ok. Was the oil pressure higher with dyno-oil?
PS: my idle is also 500 in D, but 650-700 in N or P. I try to keep the temp under 100 by shifting in 3rd when driving in the city to have the waterpump run faster. AC also works better at higher rpms. At stops I shift into N as recommended in manual to reduce heat from transmission. I can normally keep it at below 95.